Preliminary isolation of a chicken embryonic leukemic antigen has been obtained. A series of peaks on isoelectric focussing indicates the presence of multiple determinants, corresponding to the presence of species-specific cross reactivity with other avian species. Further purification of the antigen is to be obtained, from which monospecific antisera will be made. Then, the study of the ontogeny of the molecule will be undertaken, the cellular site of synthesis and its phase specificity. A study of cells of the adult will detect the possible presence of a reservoir of "stem" cells. These presumably are required for leukemic transformation, since it cannot occur in their absence.